This invention relates to an allophanate-modified, partially trimerized organic diisocyanate having cycloaliphatically bound isocyanate groups (preferably isophorone diisocyanate) which is characterized by a low Tg, and to a process of preparing this allophanate-modified, partially trimerized organic diisocyanate having cycloaliphatically bound isocyanate groups (preferably isophorone diisocyanate) which is characterized by a low Tg.
The production of isocyanurate polyisocyanates is known and described in, for example, U.S. Pat. Nos. 3,252,942, 3,487,080, 4,324,879, 4,412,073, 4,487,928, 4,537,961, 4,604,418, and 4,675,401, and DE-OS 3,240,613. Some of these prior publications disclose the use of subequivalent quantities of compounds containing hydroxyl groups. The use of polyhydric alcohols having a molecular weight below 3000 in a quantity of up to 15 mole % (based on the HDI used) in the production of isocyanurate polyisocyanates based on HDI is disclosed by U.S. Pat. No. 4,604,418. Suitable polyhydric alcohols described therein include unspecified polyester polyols. In these prior-published processes, the object of the urethane modification is merely to provide a suitable solvent for the catalyst, to achieve suitable co-catalysis or to establish compatibility with various polyols.
U.S. Pat. Nos. 5,124,427, 5,208,334, 5,235,018 and 5,444,146 disclose polyisocyanates containing allophanate and isocyanurate groups, a process for their production, and the use of these materials in two-component coating compositions. These polyisocyanate mixtures have NCO group contents of 10 to 47% by weight and contain isocyanurate and allophanate groups in a molar ratio of monoisocyanurate to monoallophanate of 10:1 to 1:5. These are prepared by catalytically trimerizing a portion of the isocyanate groups of a (cyclo)aliphatic diisocyanate, adding the monoalcohol to the diisocyanate prior to or during the trimerization reaction, and terminating the trimerization reaction at the desired degree of the reaction by adding a catalyst poison or by thermally deactivating the catalyst.
U.S. Pat. No. 5,076,958 also discloses a process for the production of isocyanurate polyisocyanates. More specifically, this process requires (a) trimerizing a portion of the isocyanate groups in the presence of a catalyst, (b) terminating the trimerization reaction, and (c) removing unreacted starting diisocyanate, and additionally, prior to step (c) adding at least one diol containing ester groups and having an average molecular weight of 350 to 950. Suitable starting diisocyanates are aliphatic or cycloaliphatic diisocyanates. This process also requires that the type of reactants used and quantitative ratios of the reactants be such that on completion of the reaction at least 10% by weight of unreacted starting diisocyanate is present in the reaction mixture (not including any inert solvent) and the molar ratio of isocyanurate groups to urethane groups in the product is 20:1 to 0.2:1. Hexamethylene diisocyanate is used in all of the working examples of U.S. Pat. No. 5,076,958.
Isocyanates that contain both allophanate groups and trimer (or isocyanurate) groups are known and described in, for example, U.S. Pat. Nos. 6,028,158 and 6,063,891, and U.S. Published Applications 20050101754 and 20070129526. These are all, however, specific to aromatic diisocyanates such as toluene diisocyanate and diphenylmethane diisocyanate.
Aliphatic diisocyanates are typically used for two-component coatings, particularly in the automotive paint industry. A blend of hexamethylene diisocyanate trimers and isophorone diisocyanate trimers is typically used in two-component automotive paint systems. Although these blends provide the desired properties, they are difficult to prepare and to use.
One of the main difficulties with blends of isocyanates such as HDI trimers and IPDI trimers is that these are difficult to catalyze due to the difference in reactivity of HDI vs. IPDI. If the HDI trimer is properly catalyzed, then the IPDI trimer typically remains unreacted. This unreacted IPDI trimer inherently reduces the chemical resistance of the system. If the IPDI trimer is properly catalyzed, then the HDI trimer reacts too fast which results in poor application. Unfortunately, there is no way to add appropriate catalysts for both HDI trimers and IPDI trimers in one system due to the differences in reactivity and Tg of the two crosslinkers.
Accordingly, there is a need in this area for a crosslinker that has a glass transition temperature (Tg) between that of the trimer of HDI and that of the trimer of IPDI. Unfortunately, there are currently no commercially available materials that have an intermediate Tg.
There are several reasons why it has it has been difficult to form a crosslinker with an intermediate Tg. The production of a material having an intermediate Tg would require that a blend of monomers be used. The use of two or more different monomers would make it difficult to control the reaction and obtain the desired blend due to differences in reactivity of the monomers. Such a process would result in a mixed monomer stream that is difficult and expensive to handle in production. In addition, the overall expense of the production process would increase due to the necessary extra tanks, segregation of stripped monomer, etc. Thus, the need for a crosslinker having an intermediate Tg still exists.
Surprisingly, the novel allophanate-modified, partially trimerized cycloaliphatic diisocyanates described herein were found to have relatively low glass transition temperatures (Tg). In particular, these allophanate-modified, partially trimerized organic diisocyanate having cycloaliphatically bound isocyanate groups (preferably isophorone diisocyanates) have a Tg ranging from about −30° C. to about 40° C. In addition, these trimer/allophanates of cycloaliphatic diisocyanates, and particularly of IPDI, exhibit long pot life and short dry times, but have comparable chemical and weathering resistance to standard blended systems (of HDI and IPDI trimers). The surprisingly favorable pot life and dry time obtained are due to the ability to catalyze without having to adjust for the different reactivities of the blend of HDI and IPDI.